Many implanted medical devices that are powered by electrical energy have been developed. Some of these devices comprise a power source, one or more conductors, and a load. Others of these implantable medical device are passive, with no active energy source.
When a patient with one of these implanted devices is subjected to high intensity magnetic fields, on the order of at least about 1 Tesla, currents are often induced in the implanted conductors. The large current flows so induced often create substantial amounts of heat. Because living organisms can generally only survive within a relatively narrow range of temperatures, these large current flows are dangerous.
Furthermore, implantable devices, such as implantable pulse generators (IPGs) and cardioverter/defibrillator/pacemaker (CDPs), are sensitive to a variety of forms of electromagnetic interference (EMI). These devices include sensing and logic systems that respond to low-level signals from the heart. Because the sensing systems and conductive elements of these implantable devices are responsive to changes in local electromagnetic fields, they are vulnerable to external sources of severe electromagnetic noise, and in particular to electromagnetic fields emitted during magnetic resonance imaging (MRI) procedures. Therefore, patients with implantable devices are generally advised not to undergo magnetic resonance imaging (MRI) procedures, which often generate static magnetic fields of from between about 0.5 to about 10 Teslas and corresponding time-varying magnetic fields of about 20 megahertz to about 430 megahertz, as dictated by the Lamor frequency (see, e.g., page 1007 of Joseph D. Bronzino's “The Biomedical Engineering Handbook,” CRC Press, Hartford, Conn., 1995). Typically, the strength of the magnetic component of such a time-varying magnetic field is about 1 to about 1,000 microTesla. In addition to the aforementioned static magnetic field and radio frequency magnetic fields, the use of MRI procedures also produces a magnetic gradient field, which allows for spatial resolution required by the MRI diagnosis.
One additional problem with conductors implanted within a living biological organism is that, when they are conducting electricity and are simultaneously subjected to large magnetic fields, a Lorentz force is created which often causes the conductor to move. This movement may damage body tissue.
Furthermore, the MRI procedures often induce a voltage within the living biological organism. This induced voltage is caused by the change of magnetic flux as a function of time. Thus, when the implanted conductor and the body tissue/fluid with which it is in electrical connection form a loop, this loop occupies an area within the body. The change of magnetic flux within such area induces a voltage which often is on the order of from about 0.1 to about 1.0 volts. As little as 0.4 volts is often sufficient to excite muscle (and other) cells and cause them to involuntarily move and/or react, a consequence which is often undesirable.
Several attempts have been made to provide effective shielding. Thus, e.g., in U.S. Pat. No. 6,265,466 an electromagnetic shielding composite having nanotubes is disclosed. This shield is adapted to shield relatively weak alternating electromagnetic fields with magnetic fields strengths of less than about 10 Gauss; but there is no disclosure that such shields would be effective against larger magnetic fields of at least about 0.5 Tesla.
It is an object of this invention to provide an implantable device which is at least partially shielded against strong magnetic fields.